Polyethylene terephthalate resin bottles (PET bottles), obtained by biaxially stretching and blow molding an injection molded preform in a test tube shape, have high transparency, high mechanical strength, and a high gas barrier property, and find their way into various fields such as beverages, foods, and cosmetics. There are many cases where the quality of a content fluid has to be prevented from decreasing, and there are use applications especially requiring a gas barrier property against oxygen and other gases. In such cases, a 2-resin/3-layer laminar structure is used for the bottle in which an intermediate layer made of, e.g., a nylon resin having a high gas barrier property is laminated with the main-resin layers of a PET resin that makes up the shape of the preform. The PET bottle having a laminar structure of this type can be molded by biaxially stretching and blow molding a preform having a test tube shape and a 2-resin/3-layer laminar structure.
Patent document 1 describes an invention associated with a molding device having a multi-layer nozzle for injection molding a 2-resin/3-layer preform such as described above. As an example, FIG. 10A shows a prior-art preform 101 of this type. FIG. 11 is a schematic vertical section of a nozzle section 11 in a device for molding the preform 101. FIG. 12 is an explanatory diagram showing an example of the injection pattern used with this molding device to mold the preform 101. FIG. 13 is an explanatory diagram showing the filling steps in which the mold cavity is filled with molten resins.
The preform 101 in FIG. 10A has a gas barrier resin layer 101b made of a resin having a high gas barrier property as an intermediate layer and laminated with the main-resin layers. This preform 101 is biaxially stretched and blow molded into a bottle having a function that is difficult for a single PET resin to achieve, for example, a function of controlling the oxidative degradation of the content by minimizing the volume of outside oxygen that penetrates the bottle. Such a bottle can be made from the preform 101.
The preform 101 having such a laminar structure is molded by using an injection pattern shown in FIG. 12 and a molding device having a multi-layer nozzle section 11 shown in FIG. 11. Now referring to the device of FIG. 11, a PET resin supplied from a first feeder Sa passes through an outer flow channel 15a and an inner flow channel 15c, and flows into a joined flow channel 19. A barrier resin supplied from a second feeder Sb passes through a middle flow channel 15b and enters the joined flow channel 19. At that time, the barrier resin Rb is put between the outer flow channel 15a and the inner flow channel 15c. Inside the joined flow channel 19, a joined resin fluid is formed in which the barrier resin Rb is laminated with the main resin Ra in a cylindrical shape. This joined resin fluid is then injected into a cavity 4 of a mold 1 to fill the cavity.
An example of injection molding is described, now referring to FIGS. 12 and 13. FIG. 13A shows a state right before point E in the injection pattern shown in FIG. 12. In this state, it is found that only the PET resin Ra have been filled. At point E, the injection of the barrier resin Rb is started. Between point E and point F, the barrier resin Rb is sandwiched between layers of the PET resin Ra, and is sent to the mold cavity in that laminated state (See FIG. 13B. At point F, the injection of the barrier resin is terminated, and from then on, the cavity is filled only with the PET resin Ra until the end of the injection step (See FIG. 13C). Thus, the preform 101 shown in FIG. 10A can be obtained.